The entire disclosure of Japanese Patent Applications Nos. 2002-82775 filed on Mar. 25, 2002 and 2002-155531 filed on May 29, 2002 including specification, drawings and abstract is herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a crankshaft and an engine including the crankshaft.
2. Description of the Related Art
Conventionally, a crankshaft installed in an engine converts the reciprocation of a piston 6 into its own rotation through a connecting rod 4, as shown in FIG. 18. Specifically, at first, the reciprocation of the piston 6 is transmitted to the connecting rod 4. Then, a crankpin 1, supporting a big end of the connecting rod 4 via a sliding bearing 5, of the crankshaft revolves around a crank journal, whereby the crankshaft rotates.
In general, a four-stroke engine works through the intake stroke, the compression stroke, the explosion stroke and the exhaust stroke as its one cycle. During each stroke, the point changes where the maximum load acts on the crankpin 1. For example, at the top dead center (T.D.C.) in the compression stroke, the top side of the crankpin 1 (close to the piston 6) indicated at a point B in FIG. 18 is loaded with the maximum explosion force. At the bottom dead center (B.D.C.) in the intake and the exhaust strokes, the top side of the crankpin 1 (close to the piston 6) indicated at the point B in FIG. 18 is loaded the maximum inertia force in the stroke. At the T.D.C. in the intake stroke, the bottom side of the crankpin 1 (opposite to the piston 6) indicated at a point A in FIG. 18 is loaded with the maximum inertia force in the stroke. Taking into consideration the inertia force loaded onto the bearing 5 in connection with the revolution of the crankpin 1, the relative movement between the bearing 5 and the crankpin 1 changes to oppose each other in the moving direction of the piston 6 at the T.D.C. and B.D.C.
As the above, during each stroke, the point loaded with the maximum force moves from the top side close to the piston 6 of the crankpin 1 to the bottom side opposite to the piston 6, and vice versa, for example. As a result, changes occur in the oil film thickness at a bearing clearance between the crankpin 1 and the sliding bearing 5. Especially, at the point A of the T.D.C. in the intake stroke and the point B of the B.D.C. in the intake stroke etc., the clearance between the crankpin 1 and the sliding bearing 5 is decreased with the high load, whereby the oil film therebetween may be sheared. As the oil film becomes thin, shear resistance increases. Further, in the case of the oil film being sheared, the power loss of the engine increases since the friction resistance increases between the crankpin 1 and the bearing 5 of the big end of the connecting rod 4. The power loss amounts to hydraulic loss as the resistance therebetween. Specifically, the power loss is the sum of the shear loss caused by rotational hydraulic resistance and the squeeze loss caused by the hydraulic compression loss which is attributed to the deviation of the bearing axis from the crankpin axis. The less the oil film thickness becomes, the larger the shear loss increases, and the larger the bearing 5 moves, the larger the squeeze loss increases.
Generally, if the power loss is less, the fuel efficiency is higher. With the recent requirement for the higher fuel efficiency, the power loss has been required to be reduced. For smoother rotation of the engine, it has been practice to grind the crankpin 1 as round as possible.
Accordingly, an object of the present invention is to provide an improved crankshaft capable of resolve the problem on the power loss of the engine.
In order to achieve the object, the inventors of the present invention have been studied and made many try-and-errors, and finally thought of making the sectional profile of the crankshaft non-circular.
A crankshaft according to the present invention comprises a crank journal, a crankpin and a crank arm. The crankpin takes an approximately cylindrical form with the sectional profile characterized below. The sectional profile of the crankpin is encircled and circumscribed by a hypothetical circle. Plural crescent spaces are provided between the sectional profile of the crankpin and the hypothetical circle. Each crescent space is an area surrounded by a part of the sectional profile of the crankpin and a part of the arc of the hypothetical circle wherein the parts are defined between two adjacent contact points.
The crescent space between the sectional profile of the crankpin and the hypothetical circle varies the radial clearance between the crankpin and a bearing attached thereon as the point on the part of the crankpin moves angularly. This advantageously makes the power loss reduce in comparison with in the case of a true circular crankpin, where the crankshaft is installed to an engine.
Further, the point where the radial clearance is the largest in each of the crescent spaces is offset angularly from the radial axis of the crank arm and is located at around the middle between two adjacent contact points.
When the crankshaft is incorporated to the engine, the point where the clearance is the largest is offset angularly from the reciprocating axis of a piston which is at the top dead center or the bottom dead center. This also advantageously makes the power loss reduce in comparison with in the case of the true circular crankpin.
Furthermore, the number of the crescent spaces is determined six at the most. Where the number of the crescent spaces is two, the point having the largest radial clearance advances 45 degrees from the radial axis of the crank arm in the rotational direction of the crank journal. Then, the point advances 30 degrees, 45 degrees and 30 degrees, in the case of three crescent spaces, four crescent spaces and six crescent spaces, respectively. These designs favorably resolve in reducing the power loss, compared with in the case of the true circular crankpin.
Moreover, the sectional profile of the crankpin is designed to a polygon which is formed by connecting plural arcs of curvature in series. These designs also favorably resolve in reducing the power loss, compared with in the case of the true circular crankpin.